Scientists have caused a fresh BSE scare in Britain following the publication of new research which suggests that the disease might be transmitted between species more easily than originally thought.
It also suggests there may be a "sub-clinical" form of BSE during which a human or animal carrier could spread the disease without displaying any of the symptoms.
The findings, published in the current issue of the US Proceedings of the National Academy of Sciences, raises the spectre of this frightening, incurable disease stalking the land unseen and unheralded.
Not surprisingly, the report has caused consternation in Britain, which has the worst BSE record in Europe and has had more than 70 deaths from the human form of the disease, variant CJD.
The work was done by the UK Medical Research Council Prion Unit, lead by the leading BSE research specialist, Prof John Collinge. A member of the British government's Spongiform Encephalopathy Advisory Com mittee, Prof Collinge pulled no punches in the report.
He warned that not just cattle but many other species, including poultry, sheep and pigs, were exposed to BSE-contaminated feed, protein foods derived from animal carcass waste. These, too, could pose a risk to humans as a result.
His research focus was on the "species barrier", which it was thought served as a natural protectant, preventing the disease jumping from one species to another.
He showed that the barrier was not absolute by extracting infected brain tissue from a hamster and injecting it into the brain of a mouse. The mouse was later shown to have the trademark biochemical signs of spongiform encephalopathy - aberrant prion proteins - but carried the disease with no apparent symptoms.
Prof Collinge then took material from the mouse brain and went on to infect other mice, who developed the full-blown disease and died.
He used extreme measures, but the results enabled him to suggest three things: that the species barrier could be broken more easily than was thought; that an infected animal might remain subclinical, free of symptoms but able to infect other animals; and what was true for one species, say cattle, could hold for others, including humans.
The results prompted him to suggest that the regulatory authorities might want to review safety considerations and human health implications. He was careful, however, to add that the existing regulatory and protective framework in Britain was sufficient to safeguard the public.
"The controls recommended by the SEAC with respect to the protection of human and animal health have always been on the basis that there were likely to be undetected cases of infection in the cattle and human populations," he said. In other words they were geared to expect the unexpected.
The possibility of inducing infection across the species barrier is no surprise, however, according to Ireland's leading BSE researcher, Dr Mark Rogers, of UCD. This is especially true given the extreme methods employed to transfer infection from the hamster to the mouse.
Equally, the long incubation period for the disease meant infected animals would in many cases have died of natural causes long before there were clinical symptoms of a spongiform encephalopathy. In effect, they would be symptom-free but infectious.
The fact that there was no immediate move to tighten up regulatory controls also suggested that the researchers had not uncovered a genuine new risk, Prof William Hall, director of the Virus Reference Laboratory at UCD, said.
He is also the chairman of the CJD advisory group, set up in September 1996 to advise the Minister for Health and Children on human forms of the disease.
Even so, the new research does propose at least a theoretical risk, particularly if there is a sub-clinical phase of the disease. It would be during this time that disease could be transferred. The regulatory bodies in Ireland have considered these possibilities, however, and the consumer should be protected even from infected but undetected animals.
"We have a comprehensive list of controls that have been in place since the early 1990s," said a spokesman for the Department of Agriculture, Food and Rural Development.
Prof Collinge suggested in his report that his work "raises the possibility that apparently healthy cattle could harbour, but never show signs of, BSE".
The Department's response is "slaughterout". If a BSE-infected animal is discovered, the entire herd with which it lived and its siblings are slaughtered and disposed of. "That slaughterout policy has been in place since the start," the spokesman said.
The assumption is also made here that any animal might be infected so the "specified risk material", tissues such as spinal cord, brain and certain organs where BSE-related prion proteins are found, is removed from all cattle and sheep sent for processing and disposed of separately.
An additional sampling procedure had been introduced since the beginning of July, the spokesman said. Animals were selected at random and analysed for the presence of BSE prions or signs of BSE in the brain.
Prof Collinge's research also raises the possibility that there could be a human carrier of the disease. The risk here is that if this person required surgical intervention for any reason, the surgical tools might allow the transfer of the disease to the next patient.
Prof Hall's advisory group drew up best-practice procedures and recommendations to minimise the risk of such transfers, however, and these were circulated to health boards and hospitals.